package org.apache.solr.analysis;

import java.io.IOException;
import org.apache.lucene.analysis.TokenFilter;
import org.apache.lucene.analysis.TokenStream;
import org.apache.lucene.analysis.tokenattributes.CharTermAttribute;
import org.apache.lucene.analysis.tokenattributes.PositionIncrementAttribute;

/**
 * This class produces a special form of reversed tokens, suitable for
 * better handling of leading wildcards. Tokens from the input TokenStream
 * are reversed and prepended with a special "reversed" marker character.
 * If <code>withOriginal<code> argument is <code>true</code> then first the
 * original token is returned, and then the reversed token (with
 * <code>positionIncrement == 0</code>) is returned. Otherwise only reversed
 * tokens are returned.
 * <p>Note: this filter doubles the number of tokens in the input stream when
 * <code>withOriginal == true</code>, which proportionally increases the size
 * of postings and term dictionary in the index.
 */
public final class ReversedWildcardFilter extends TokenFilter {

    private boolean withOriginal;
    private char markerChar;
    private State save;
    private CharTermAttribute termAtt;
    private PositionIncrementAttribute posAtt;

    protected ReversedWildcardFilter(TokenStream input, boolean withOriginal, char markerChar) {

        super(input);
        this.termAtt = addAttribute(CharTermAttribute.class);
        this.posAtt = addAttribute(PositionIncrementAttribute.class);
        this.withOriginal = withOriginal;
        this.markerChar = markerChar;
    }

    @Override
    public boolean incrementToken() throws IOException {

        if (save != null) {
            // clearAttributes();  // not currently necessary
            restoreState(save);
            save = null;
            return true;
        }

        if (!input.incrementToken()) {
            return false;
        }

        // pass through zero-length terms
        int oldLen = termAtt.length();
        if (oldLen == 0) {
            return true;
        }
        int origOffset = posAtt.getPositionIncrement();
        if (withOriginal == true) {
            posAtt.setPositionIncrement(0);
            save = captureState();
        }
        char[] buffer = termAtt.resizeBuffer(oldLen + 1);
        buffer[oldLen] = markerChar;
        reverse(buffer, 0, oldLen + 1);

        posAtt.setPositionIncrement(origOffset);
        termAtt.copyBuffer(buffer, 0, oldLen + 1);
        return true;
    }

    /**
     * Partially reverses the given input buffer in-place from the given offset
     * up to the given length, keeping surrogate pairs in the correct
     * (non-reversed) order.
     *
     * @param buffer the input char array to reverse
     * @param start the offset from where to reverse the buffer
     * @param len the length in the buffer up to where the buffer should be
     * reversed
     */
    public static void reverse(final char[] buffer, final int start, final int len) {
        /* modified version of Apache Harmony AbstractStringBuilder reverse0() */
        if (len < 2) {
            return;
        }
        int end = (start + len) - 1;
        char frontHigh = buffer[start];
        char endLow = buffer[end];
        boolean allowFrontSur = true, allowEndSur = true;
        final int mid = start + (len >> 1);
        for (int i = start; i < mid; ++i, --end) {
            final char frontLow = buffer[i + 1];
            final char endHigh = buffer[end - 1];
            final boolean surAtFront = allowFrontSur && Character.isSurrogatePair(frontHigh, frontLow);
            if (surAtFront && (len < 3)) {
                // nothing to do since surAtFront is allowed and 1 char left
                return;
            }
            final boolean surAtEnd = allowEndSur && Character.isSurrogatePair(endHigh, endLow);
            allowFrontSur = allowEndSur = true;
            if (surAtFront == surAtEnd) {
                if (surAtFront) {
                    // both surrogates
                    buffer[end] = frontLow;
                    buffer[--end] = frontHigh;
                    buffer[i] = endHigh;
                    buffer[++i] = endLow;
                    frontHigh = buffer[i + 1];
                    endLow = buffer[end - 1];
                }
                else {
                    // neither surrogates
                    buffer[end] = frontHigh;
                    buffer[i] = endLow;
                    frontHigh = frontLow;
                    endLow = endHigh;
                }
            }
            else {
                if (surAtFront) {
                    // surrogate only at the front
                    buffer[end] = frontLow;
                    buffer[i] = endLow;
                    endLow = endHigh;
                    allowFrontSur = false;
                }
                else {
                    // surrogate only at the end
                    buffer[end] = frontHigh;
                    buffer[i] = endHigh;
                    frontHigh = frontLow;
                    allowEndSur = false;
                }
            }
        }
        if ((len & 0x01) == 1 && !(allowFrontSur && allowEndSur)) {
            // only if odd length
            buffer[end] = allowFrontSur ? endLow : frontHigh;
        }
    }
}
